This invention relates to devices for holding work pieces during machining, and more specifically to a chuck for opening and closing the leaves on the end of a split collet which centers and holds the work piece to be machined.
In certain machining operations, such as those utilizing a lathe having a hollow spindle, a split collet held by a chuck is used to securely grasp the work piece. In conventional practice, a draw tube inserted into one end of the hollow spindle is connected to an end of the split collet. The draw tube is pulled rearward to force the split end of the collet against the chuck so as to bias the leaves of the collet together, thereby centering and securing the work piece. Typically, the draw tube is pulled rearward by either a hand wheel or a lever located near the end of the hollow spindle. In order to release the work piece after machining, the draw tube is moved forward to move the leaves on the split end of the collet away from the chuck, thereby loosening the work piece within the collet
The draw tube type of collet closing devices are cumbersome and, since the hand wheel or lever is located some distance from the work piece, the machinist must move to the end of the lathe to open or close the collet. Additionally, such devices are not capable for use on machines other then lathes, such as vertical milling machines.
U.S. Letters Pat. No. 3,055,671 discloses a quick-release collet closing device. This patent discloses a spring-biased split ring having grooves on its radially inner surface. The forward face of the ring has cams which are adapted to contact rearwardly oriented cams on the back of the chuck. The angular movement of a lever arm projecting radially outwardly from the chuck and connected to the split ring simultaneously forces the ring halves together to grip the end of the collet and causes the cam surfaces on the ring to contact the cam surfaces on the chuck, thereby displacing the collet rearwardly, forcing the leaves on the split end of the collet against the chuck, and biasing the leaves together to grip the work piece.
This type of chuck has inadequate provision for locking the collet in either the open or the closed position. Thus when the work piece is inserted into the open collet, The collet must be held in the chuck to prevent the end of the collet from moving out of alignment with the split ring. When the collet is closed about the work piece, the only forces acting to keep the collet closed are axial forces generated by the opposing cam surfaces. The split ring is not locked from rotating so that any disturbing torques generated by the machining operation may cause the split ring to rotate, the ring halves to separate, and the collet and work piece to become loose in the chuck. Additionally, this type of chuck will not accept collets having threaded ends and is complicated to operate in that it requires a first angular movement of the lever arm to grip the collet and a further angular movement to engage the cam surfaces on the ring with those on the chuck.